1. Field of the Invention
The present invention relates to an ink, particularly to an ink for ink jet recording, and more particularly to an ink, an ink set, an ink jet recording method, an ink jet recording apparatus, a recording unit and an ink cartridge suitable for thermal ink jet recording system, that can provide a recorded matter with little deterioration of image quality over an prolonged time period.
2. Related Background Art
Recently, studies of ink jet ink containing pigment as a coloring material have been energetically conducted to meet the increasing need for further improved image fastness (light fastness, gas fastness, etc.).
As a result, the immediate problems of pigment ink-jet inks, i.e., nozzle clogging in the head and long-term storage instability of the ink, are considerably improved.
Meanwhile, in order to obtain high-grade photographic images by ink jet recording using ink containing a water-soluble coloring material such as a dye, there is a method to use light ink in addition to dark color inks (for example, yellow (Y) ink, cyan (C) ink, magenta (M) ink and black (Bk) ink), where light ink is an ink that forms images of the same color hue as one of the dark inks does but has a lower coloring power than the corresponding dark ink. Usually, light cyan ink and light magenta ink are used as light ink.
Also in the ink jet recording system using inks containing a water-insoluble coloring material such as pigment inks, has been investigated a combination of two inks that provide images of the same color but are different in coloring power, that is, a combination of a dark ink and a light ink. For example, WO 01/48100 discloses an ink set of a dark ink and a light ink both containing a pigment and a dispersant where the ratio (B1/P1) of the resin content (B1) to the pigment content (P1) in the dark ink is smaller than the ratio (B2/P2) of the resin content (B2) to the pigment content (P2) in the light ink and B1 and B2 are different. This provides an ink set that shows a uniform permeability of the ink and gives a high quality image with no surface roughness. Further, by setting B1 larger B2, the light fastness of the light ink can be improved to a level almost equivalent to the light fastness of the dark ink. In addition, Japanese Patent Application Laid-Open No. 2001-179956 discloses a technology for improving the roughness of the images recorded with a dark ink and a light ink both containing a pigment and a dispersant, by rendering the surface tension of the light ink lower than that of the dark ink.